Terrible how? Races or battles?
The main way to improve is to keep playing over and over. Try using different combinations of character/vehicle until you find one that suits your style of driving. My personal preference for races is the Magikruzer and Baby Mario although I'm also warming to Mario on the Dolphin Dasher .. .. .. ..
If you can then play online because playing against other human players is harder than against the AI. Watch what others do, where they powerslide, where they wheelie the bikes, which lines they take through corners.
Learn how to use the weapons effectively. Timing is important.